It is known that an optical lens (convex lens) shows a characteristic that a refractive index with short-wavelength light is higher than that with long-wavelength light, i.e. axial chromatic aberration, and has color bleeding (chromatic aberration). Since the chromatic aberration is a problem in using the optical lens, the optical lens is generally used as a lens system or laminated lens having chromatic aberration corrected by combining achromatic elements.
As the achromatic element, a concave lens has been used. In recent years, use of a Fresnel lens has been receiving attention (refer to JP-A 9-127321, JP-A 9-127322, JP-A 2004-78166 and JP-A 2005-107298)(the term “JP-A” as used herein means an “unexamined published Japanese patent application”).
For correction of the chromatic aberration, the lens system is constituted by a plurality of elements made of different materials whose Abbe numbers, which are indicators for the degree of the chromatic aberration, are largely different. As a material for lenses, glass has been used. Since glass has a refractive index of about 1.46 to 1.92 and an Abbe number of about 25 to 80 and the optical properties can be selected from the very wide ranges, it provides high flexibility to design of the lens system for correcting the chromatic aberration, and it is an advantageous material for exhibiting various lens properties.
When the above lens system is applied to a camera, video camera or the like, reductions in the size and weight of the lens system are required. Particularly, a telephoto lens system and super-telephoto lens system having a large lens diameter have been massive and heavy and difficult to handle when attached to a camera and used, so that one may miss a right moment to release a shutter. Accordingly, reductions in the weight and whole length of the lens system have been desired. To that end, it is necessary to reexamine the material for the lens and shorten focal length after correction of chromatic aberration.
From such a viewpoint, use of a plastic material in place of glass as the lens material has been desired.
Typical materials for the current plastic lenses are polymethyl methacrylate (PMMA) and polycarbonate. Of these, the PMMA is used far more often, because it not only has advantages that it has high transparency and hardness and excellent durability but also has a high Abbe number of 57 or small chromatic aberration.
As a material for a plastic lens used for correction of chromatic aberration of a PMMA lens, polycarbonate (PC(BIS-A)) produced from 2,2-bis(4-hydroxyphenyl)propane(bisphenol A) is generally used. The Abbe number of the PC(BIS-A) is 32 and greatly differs from the Abbe number of the PMMA which is 57. Thus, it supposedly has the requirement for correction of chromatic aberration. However, even with the PC(BIS-A), the focal length after correction of chromatic aberration has not yet reached a satisfactory level.
Meanwhile, it has been considered that a higher refractive index of a lens material is more advantageous from the viewpoint of a reduction in the weight of a lens. That is, it has been considered that when the refractive index is low, the curve of the lens must be made tight to obtain a desired effect, so that the entire lens must be made thick, and this is against the request of the weight reduction.
It is known that conventionally known plastic optical materials have a definite relationship between a refractive index and an Abbe number. FIG. 1 shows relationships between the refractive indices and Abbe numbers of general-purpose optical plastic materials described in known literatures. Referring to FIG. 1, it can be understood that the plastic materials show a tendency that the Abbe number becomes lower as the refractive index becomes higher and a collection of plotted points each representing the relationship between the refractive index and Abbe number of each material is situated within a very narrow range. Efforts to improve the plastic materials as conventional optical elements have been directed primarily to developments of materials showing a high refractive index exceeding the above range (refer to Japan Chemical Industry Association Monthly, 40 (8), 27 (1987) and Japan Chemical Industry Association Monthly 47 (2), 8 (1994)).
However, it has not yet been known what material is to be used as an achromatic element which is suitable for correction of chromatic aberration in a lens system and can constitute a lens system having shortened focal length after correction of chromatic aberration.
Polycarbonate materials have been known as materials having transparency and heat resistance, and many improvements and studies have been made on the materials. However, most of them are improvements of mechanical properties as engineering plastics (refer to JP-A 6-145333 and JP-A 6-200005) or are intended to use the materials for special purposes (refer to JP-A 5-294029). Most of studies in optical applications are improvements of mechanical properties in fields that involve only a single wavelength such as an optical fiber, a photoreceptor for photocopying or a recording layer of an optical disk (refer to JP-A 6-347650, JP-A 3-61911, JP-A 6-282094 and JP-A 64-31825). Only in JP-A 62-18501, improvements and studies have been made to apply polycarbonate materials to lenses. However, in this publication, material studies intended to improve the heat resistance and surface hardness of lenses have been made, and in examples, convex lenses made of the materials are merely disclosed.